I omly know up to PowerMac G4's, so all I can tell you to do applies to them whether it applies to G5's is beyond me.
Here's some ideas:
reset the PRAM: on my computer you do that by holding down P, R, Command (the Apple key) and Option right after your hear the first chime and hold til you hear the second chime then release.
Open the unit and press the rest button on the motherboard.
Hold down the Option key while you start until you get the screen that shows your hard drive, then click on that.
Put another hard drive in and leave the old one and attempt to install the Mac OS DVD on to that.
Start the thing up while you hold the T key until you get the Target Mode screen and then plug a Firewire cable from it to a working mac and then go into Disk Utility and check the permissions/disc on the broken computer.
Put a voltmeter on the battery on the motherboard and see if it's ok (
my Power Mac won't start with a dead battery).
Hold down Command while you start to enter Safe Mode.
Put the Mac OS DVD in the drive and start up while holding the C key down.
Go on line and find the test points for voltages on the power supply to see if that's ok (not as hard as it sounds).
For starters...
